There is a strong need in the telecommunications market, particularly in the area of 4G wireless communication systems, as well as in existing wireless systems, for miniature type filters with improved performance from current levels. As 4G systems target a very high speed data transfer, they need much wider bandwidth than existing systems such as GSM, CDMA and UMTS. On the other hand, limited frequency resources in 4G systems require wireless carrier companies to set guard-bands as narrow as possible to enable maximum user capacity. Combining these two issues means that the 4G wireless systems require miniature RF filters for their wireless terminal devices that not only have a wide pass band or reject-band, but also have steep transition bands.
Due to their miniature size and low cost, acoustic materials-based RF filters such as surface acoustic wave (SAW), thin film bulk acoustic resonator (FBAR) and/or bulk acoustic wave (BAW) filters are widely used in compact and portable type terminal devices of various wireless systems. However, the current level of filter performance of these filters is still far from 4G wireless system filter requirements.
Some non-acoustic microwave technology type filters, such as metal-type cavity filters or dielectric filters can be designed to meet filter performance requirements for these applications, but these types of designs have an ultra-high cost and result in physically large filters. As a result, metal-type cavity filters and dielectric filters are undesirable, particularly for applications in wireless terminals, for which size and weight are of considerable importance.